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  <div class="section" id="module-atexit">
<span id="atexit-exit-handlers"></span><h1>29.8. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal"><span class="pre">atexit</span></code></a> — Exit handlers<a class="headerlink" href="#module-atexit" title="Permalink to this headline">¶</a></h1>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal"><span class="pre">atexit</span></code></a> module defines functions to register and unregister cleanup
functions.  Functions thus registered are automatically executed upon normal
interpreter termination.  <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal"><span class="pre">atexit</span></code></a> runs these functions in the <em>reverse</em>
order in which they were registered; if you register <code class="docutils literal"><span class="pre">A</span></code>, <code class="docutils literal"><span class="pre">B</span></code>, and <code class="docutils literal"><span class="pre">C</span></code>,
at interpreter termination time they will be run in the order <code class="docutils literal"><span class="pre">C</span></code>, <code class="docutils literal"><span class="pre">B</span></code>,
<code class="docutils literal"><span class="pre">A</span></code>.</p>
<p><strong>Note:</strong> The functions registered via this module are not called when the
program is killed by a signal not handled by Python, when a Python fatal
internal error is detected, or when <a class="reference internal" href="os.html#os._exit" title="os._exit"><code class="xref py py-func docutils literal"><span class="pre">os._exit()</span></code></a> is called.</p>
<dl class="function">
<dt id="atexit.register">
<code class="descclassname">atexit.</code><code class="descname">register</code><span class="sig-paren">(</span><em>func</em>, <em>*args</em>, <em>**kargs</em><span class="sig-paren">)</span><a class="headerlink" href="#atexit.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register <em>func</em> as a function to be executed at termination.  Any optional
arguments that are to be passed to <em>func</em> must be passed as arguments to
<a class="reference internal" href="#atexit.register" title="atexit.register"><code class="xref py py-func docutils literal"><span class="pre">register()</span></code></a>.  It is possible to register the same function and arguments
more than once.</p>
<p>At normal program termination (for instance, if <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal"><span class="pre">sys.exit()</span></code></a> is called or
the main module’s execution completes), all functions registered are called in
last in, first out order.  The assumption is that lower level modules will
normally be imported before higher level modules and thus must be cleaned up
later.</p>
<p>If an exception is raised during execution of the exit handlers, a traceback is
printed (unless <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></code></a> is raised) and the exception information is
saved.  After all exit handlers have had a chance to run the last exception to
be raised is re-raised.</p>
<p>This function returns <em>func</em>, which makes it possible to use it as a
decorator.</p>
</dd></dl>

<dl class="function">
<dt id="atexit.unregister">
<code class="descclassname">atexit.</code><code class="descname">unregister</code><span class="sig-paren">(</span><em>func</em><span class="sig-paren">)</span><a class="headerlink" href="#atexit.unregister" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove <em>func</em> from the list of functions to be run at interpreter
shutdown.  After calling <a class="reference internal" href="#atexit.unregister" title="atexit.unregister"><code class="xref py py-func docutils literal"><span class="pre">unregister()</span></code></a>, <em>func</em> is guaranteed not to be
called when the interpreter shuts down, even if it was registered more than
once.  <a class="reference internal" href="#atexit.unregister" title="atexit.unregister"><code class="xref py py-func docutils literal"><span class="pre">unregister()</span></code></a> silently does nothing if <em>func</em> was not previously
registered.</p>
</dd></dl>

<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">readline</span></code></a></dt>
<dd>Useful example of <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal"><span class="pre">atexit</span></code></a> to read and write <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">readline</span></code></a> history
files.</dd>
</dl>
</div>
<div class="section" id="atexit-example">
<span id="id1"></span><h2>29.8.1. <a class="reference internal" href="#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal"><span class="pre">atexit</span></code></a> Example<a class="headerlink" href="#atexit-example" title="Permalink to this headline">¶</a></h2>
<p>The following simple example demonstrates how a module can initialize a counter
from a file when it is imported and save the counter’s updated value
automatically when the program terminates without relying on the application
making an explicit call into this module at termination.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
    <span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;counterfile&quot;</span><span class="p">)</span> <span class="k">as</span> <span class="n">infile</span><span class="p">:</span>
        <span class="n">_count</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">infile</span><span class="o">.</span><span class="n">read</span><span class="p">())</span>
<span class="k">except</span> <span class="ne">FileNotFoundError</span><span class="p">:</span>
    <span class="n">_count</span> <span class="o">=</span> <span class="mi">0</span>

<span class="k">def</span> <span class="nf">incrcounter</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
    <span class="k">global</span> <span class="n">_count</span>
    <span class="n">_count</span> <span class="o">=</span> <span class="n">_count</span> <span class="o">+</span> <span class="n">n</span>

<span class="k">def</span> <span class="nf">savecounter</span><span class="p">():</span>
    <span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;counterfile&quot;</span><span class="p">,</span> <span class="s2">&quot;w&quot;</span><span class="p">)</span> <span class="k">as</span> <span class="n">outfile</span><span class="p">:</span>
        <span class="n">outfile</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s2">&quot;</span><span class="si">%d</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">_count</span><span class="p">)</span>

<span class="kn">import</span> <span class="nn">atexit</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">savecounter</span><span class="p">)</span>
</pre></div>
</div>
<p>Positional and keyword arguments may also be passed to <a class="reference internal" href="#atexit.register" title="atexit.register"><code class="xref py py-func docutils literal"><span class="pre">register()</span></code></a> to be
passed along to the registered function when it is called:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">goodbye</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">adjective</span><span class="p">):</span>
    <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Goodbye, </span><span class="si">%s</span><span class="s1">, it was </span><span class="si">%s</span><span class="s1"> to meet you.&#39;</span> <span class="o">%</span> <span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">adjective</span><span class="p">))</span>

<span class="kn">import</span> <span class="nn">atexit</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">goodbye</span><span class="p">,</span> <span class="s1">&#39;Donny&#39;</span><span class="p">,</span> <span class="s1">&#39;nice&#39;</span><span class="p">)</span>

<span class="c1"># or:</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">goodbye</span><span class="p">,</span> <span class="n">adjective</span><span class="o">=</span><span class="s1">&#39;nice&#39;</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">&#39;Donny&#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>Usage as a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">atexit</span>

<span class="nd">@atexit</span><span class="o">.</span><span class="n">register</span>
<span class="k">def</span> <span class="nf">goodbye</span><span class="p">():</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;You are now leaving the Python sector.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>This only works with functions that can be called without arguments.</p>
</div>
</div>


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